Display device and electronic device

ABSTRACT

A display device where the influence of variations in current of the light emitting element due to changes in ambient temperature and changes with time can be suppressed. The display device of the invention has a light emitting element, a driving transistor connected in series to the light emitting element, a monitoring light emitting element, a limiter transistor connected in series to the monitoring light emitting element, a constant current source for supplying a constant current to the monitoring light emitting element, and a circuit for outputting a potential equal to an inputted potential. A first electrode of the light emitting element is connected to an output terminal of the circuit through the driving transistor, and a first electrode of the monitoring light emitting element is connected to an input terminal of the circuit through the limiter transistor. The channel length L 1  and the channel width W 1  of the driving transistor, and the channel length L 2  and the channel width W 2  of the limiter transistor satisfy L 1 /W 1 : L 2 /W 2 =1:2 to 1:10.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device and a televisiondevice each having a light emitting element.

The invention further relates to an electronic device that uses adisplay device having a light emitting element.

2. Description of the Related Art

In recent years, a display device having a light emitting elementtypified by an EL (Electroluminescence) element has been activelydeveloped, and it is expected to be widely used by taking advantages ofthe light emitting element such as high image quality, wide viewingangle, and reduced thickness and weight.

The light emitting element has the characteristic that its resistance(internal resistance) varies with the surrounding temperature(hereinafter referred to as ambient temperature). Specifically, theresistance decreases when the temperature is more than room temperature,while the resistance increases when the temperature is less than roomtemperature. Such a characteristic of the light emitting element isshown in a graph of FIG. 10A showing the relation betweenvoltage-current characteristics of the light emitting element andtemperature. The light emitting element also has the characteristic thatits current decreases with time. Such a characteristic of the lightemitting element is shown in a graph of FIG. 10B showing the relationbetween voltage-current characteristics of the light emitting elementand time.

SUMMARY OF THE INVENTION

The aforementioned characteristics of the light emitting element causevariations in luminance with changes in ambient temperature or changeswith time. In view of the foregoing, the invention provides a displaydevice where the influence of variations in current of the lightemitting element due to changes in ambient temperature and changes withtime can be suppressed.

In view of the foregoing, the invention provides a display device havinga correction function for changes in ambient temperature and acorrection function for changes with time (hereinafter collectivelyreferred to as a correction function).

A display device of the invention has a light emitting element, adriving transistor connected in series to the light emitting element, amonitoring light emitting element, a limiter transistor connected inseries to the monitoring light emitting element, a constant currentsource for supplying a constant current to the monitoring light emittingelement, and a buffer amplifier. A first electrode of the light emittingelement is connected to an output terminal of the buffer amplifierthrough the driving transistor. A first electrode of the monitoringlight emitting element is connected to an input terminal of the bufferamplifier through the limiter transistor. The channel length (L1) andthe channel width (W1) of the driving transistor, and the channel length(L2) and the channel width (W2) of the limiter transistor satisfyL1/W1:L2/W2=1:2 to 1:10. The limiter transistor is on all the time.

A display device of the invention has a light emitting element, amonitoring light emitting element, an AC transistor connected in seriesto the monitoring light emitting element, a constant current source forsupplying a constant current to the monitoring light emitting element,and a buffer amplifier. A first electrode of the light emitting elementis electrically connected to an output terminal of the buffer amplifier.A first electrode of the monitoring light emitting element, a gateelectrode of the AC transistor, and one of a source electrode and adrain electrode of the AC transistor are connected to an input terminalof the buffer amplifier. The other of the source electrode and the drainelectrode of the AC transistor is connected to an AC power supply.

A display device of the invention has a light emitting element, amonitoring light emitting element, a constant current source forsupplying a constant current to the monitoring light emitting element, abuffer amplifier, a capacitor connected to an input terminal of thebuffer amplifier, a first switch provided between a first electrode ofthe light emitting element and an output terminal of the bufferamplifier, a second switch provided between the first electrode of thelight emitting element and an AC power supply, a third switch providedbetween a first electrode of the monitoring light emitting element andthe input terminal of the buffer amplifier, and a fourth switch providedbetween the first electrode of the monitoring light emitting element andthe AC power supply.

The display device of the invention has a control circuit for applying aforward bias voltage to the light emitting element and the monitoringlight emitting element while bringing the first switch and the thirdswitch into a conductive state whereas the second switch and the fourthswitch into a non-conductive state. The display device of the inventionfurther has a control circuit for applying a reverse bias voltage to thelight emitting element and the monitoring light emitting element whilebringing the first switch and the third switch into a non-conductivestate whereas the second switch and the fourth switch into a conductivestate.

A display device of the invention has a light emitting element, amonitoring light emitting element, a current source transistor connectedin series to the monitoring light emitting element, and a bufferamplifier. A first electrode of the light emitting element iselectrically connected to an output terminal of the buffer amplifier. Afirst electrode of the monitoring light emitting element and one of asource electrode and a drain electrode of the current source transistoris connected to an input terminal of the buffer amplifier. A gateelectrode of the current source transistor is connected to a first powersupply, and the other of the source electrode and the drain electrode ofthe current source transistor is connected to a second power supply. Thecurrent source transistor operates in a saturation region.

A display device of the invention has a light emitting element, amonitoring light emitting element, a constant current source forsupplying a constant current to the monitoring light emitting element, abuffer amplifier, and a resistor provided between a first electrode ofthe monitoring light emitting element and an input terminal of thebuffer amplifier. A first electrode of the light emitting element iselectrically connected to an output terminal of the buffer amplifier.

A display device of the invention has a light emitting element, aswitching transistor, a monitoring light emitting element, a forwardbias transistor connected in series to the monitoring light emittingelement, a constant current source for supplying a constant current tothe monitoring light emitting element, and a buffer amplifier. A firstelectrode of the light emitting element is electrically connected to anoutput terminal of the buffer amplifier. A first electrode of themonitoring light emitting element is connected to an input terminal ofthe buffer amplifier. A gate electrode of the switching transistor and agate electrode of the forward bias transistor are connected to a gateline. One of a source electrode and a drain electrode of the forwardbias transistor is connected to the input terminal of the bufferamplifier while the other thereof is connected to a forward bias powersupply.

A display device of the invention has a pixel area including a pluralityof pixels, a source driver, a first gate driver, and a second gatedriver. Each of the pixels has a light emitting element, a firsttransistor for controlling video signal input to the pixel, a secondtransistor for controlling light emission or non-light emission of thelight emitting element, and a capacitor for holding the video signal.The capacitor has a first conductive layer formed on the same layer asgate electrodes of the first transistor and the second transistor, asecond conductive layer formed on the same layer as source and drainwirings of the first transistor and the second transistor, and aninsulating layer formed between the first conductive layer and thesecond conductive layer.

A display device of the invention has a pixel area including a pluralityof pixels, a source driver, a first gate driver, and a second gatedriver. Each of the pixels has a light emitting element, a firsttransistor for controlling video signal input to the pixel, a secondtransistor for controlling light emission or non-light emission of thelight emitting element, and a capacitor for holding the video signal.The capacitor has a first conductive layer formed on the same layer assource and drain wirings of the first transistor and the secondtransistor, a second conductive layer formed on the same layer as apixel electrode of the light emitting element, and an insulating layerformed between the first conductive layer and the second conductivelayer.

The invention provides a display device having a displaying white lightemitting element (light emitting element that emits white light) as wellas a monitoring white light emitting element. Changes in ambienttemperature and changes with time are detected by the monitoring whitelight emitting element, and the result thereof is reflected in a powersupply potential of the displaying white light emitting element.

More specifically, the displaying white light emitting element operateswith a constant voltage drive while the monitoring white light emittingelement operates with a constant current drive. In the constant voltagedrive, a constant voltage is applied to the light emitting element,whereas in the constant current drive, a constant current is supplied tothe light emitting element. When the monitoring white light emittingelement operates with the constant current drive, changes in ambienttemperature and changes with time are shown as a potential difference ofthe monitoring white light emitting element. When such changes inpotential difference of the monitoring white light emitting element arereflected in a power supply potential of the displaying white lightemitting element, changes in ambient temperature and changes with timecan be corrected.

The invention provides a display device where a white light emittingelement has a duty ratio of 45 to 80% and a monitoring white lightemitting element has a duty ratio of 45 to 100%. The duty ratio of thewhite light emitting element is the average duty ratio of all the whitelight emitting elements provided in a pixel area. The duty ratio is theratio of a lighting period to a lighting period and a non-light emittingperiod such as a writing period when all the inputted video signalsdisplay white.

The invention provides a display device where the total amount ofcurrent in a white light emitting element during a certain period isless than the total amount of current in a monitoring white lightemitting element.

In this manner, the load on the white light emitting element differsfrom the load on the monitoring white light emitting element, andluminance decay is taken into consideration based on the amount ofcharge flowing through the white light emitting element. Accordingly,constant luminance drive can be performed where the amount of charge inthe white light emitting element is compared with the amount of chargein the monitoring white light emitting element, and the luminance of thewhite light emitting element is corrected so as to be constant.

A display device of the invention has a white light emitting element, adriving transistor connected in series to the white light emittingelement, a monitoring white light emitting element, a limiter transistorconnected in series to the monitoring white light emitting element, aconstant current source for supplying a constant current to themonitoring white light emitting element, a buffer amplifier, and acolored layer. A first electrode of the white light emitting element isconnected to an output terminal of the buffer amplifier through thedriving transistor. A first electrode of the monitoring white lightemitting element is connected to an input terminal of the bufferamplifier through the limiter transistor. The channel length (L1) andthe channel width (W1) of the driving transistor, and the channel length(L2) and the channel width (W2) of the limiter transistor satisfyL1/W1:L2/W2=1:2 to 1:10. The white light emitting element is provided soas to overlap the colored layer. The limiter transistor is on all thetime.

A display device of the invention has a white light emitting element, amonitoring white light emitting element, an AC transistor connected inseries to the monitoring white light emitting element, a constantcurrent source for supplying a constant current to the monitoring whitelight emitting element, a buffer amplifier, and a colored layer. A firstelectrode of the white light emitting element is electrically connectedto an output terminal of the buffer amplifier. A first electrode of themonitoring white light emitting element, a gate electrode of the ACtransistor, and one of a source electrode and a drain electrode of theAC transistor are connected to an input terminal of the bufferamplifier. The other of the source electrode and the drain electrode ofthe AC transistor is connected to an AC power supply. The white lightemitting element is provided so as to overlap the colored layer.

A display device of the invention has a white light emitting element, amonitoring white light emitting element, a constant current source forsupplying a constant current to the monitoring white light emittingelement, a buffer amplifier, a colored layer, a capacitor connected toan input terminal of the buffer amplifier, a first switch providedbetween a first electrode of the white light emitting element and anoutput terminal of the buffer amplifier, a second switch providedbetween the first electrode of the white light emitting element and anAC power supply, a third switch provided between a first electrode ofthe monitoring white light emitting element and the input terminal ofthe buffer amplifier, and a fourth switch provided between the firstelectrode of the monitoring white light emitting element and the ACpower supply. The white light emitting element is provided so as tooverlap the colored layer.

The display device of the invention has a control circuit for applying aforward bias voltage to the white light emitting element and themonitoring white light emitting element while bringing the first switchand the third switch into a conductive state whereas the second switchand the fourth switch into a non-conductive state. The display device ofthe invention further has a control circuit for applying a reverse biasvoltage to the white light emitting element and the monitoring whitelight emitting element while bringing the first switch and the thirdswitch into a non-conductive state whereas the second switch and thefourth switch into a conductive state. The white light emitting elementis provided so as to overlap the colored layer.

A display device of the invention has a white light emitting element, amonitoring white light emitting element, a current source transistorconnected in series to the monitoring white light emitting element, abuffer amplifier, and a colored layer. A first electrode of the whitelight emitting element is electrically connected to an output terminalof the buffer amplifier. A first electrode of the monitoring white lightemitting element and one of a source electrode and a drain electrode ofthe current source transistor are connected to an input terminal of thebuffer amplifier. A gate electrode of the current source transistor isconnected to a first power supply. The other of the source electrode andthe drain electrode of the current source transistor is connected to asecond power supply. The white light emitting element is provided so asto overlap the colored layer. The current source transistor operates ina saturation region.

A display device of the invention has a white light emitting element, amonitoring white light emitting element, a constant current source forsupplying a constant current to the monitoring white light emittingelement, a buffer amplifier, a colored layer, and a resistor providedbetween a first electrode of the monitoring white light emitting elementand an input terminal of the buffer amplifier. A first electrode of thewhite light emitting element is electrically connected to an outputterminal of the buffer amplifier. The white light emitting element isprovided so as to overlap the colored layer.

A display device of the invention has a white light emitting element, aswitching transistor, a monitoring white light emitting element, aforward bias transistor connected in series to the monitoring whitelight emitting element, a constant current source for supplying aconstant current to the monitoring white light emitting element, abuffer amplifier, and a colored layer. A first electrode of the whitelight emitting element is electrically connected to an output terminalof the buffer amplifier. A first electrode of the monitoring white lightemitting element is connected to an input terminal of the bufferamplifier. A gate electrode of the switching transistor and a gateelectrode of the forward bias transistor are connected to a gate line.One of a source electrode and a drain electrode of the forward biastransistor is connected to the input terminal of the buffer amplifier,and the other thereof is connected to a forward bias power supply. Thewhite light emitting element is provided so as to overlap the coloredlayer.

The invention having the aforementioned structures can provide a displaydevice where the influence of variations in current of the lightemitting element due to changes in ambient temperature and changes withtime can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a display device of theinvention.

FIGS. 2A and 2B are diagrams each showing a configuration of a displaydevice of the invention.

FIG. 3 is a diagram showing a layout of a display device of theinvention.

FIG. 4 is a diagram showing a configuration of a display device of theinvention.

FIGS. 5A and 5B are timing charts each showing operation of a displaydevice of the invention.

FIGS. 6A and 6B are diagrams each showing a configuration of a displaydevice of the invention.

FIGS. 7A and 7B are diagrams each showing a panel that is one mode ofthe display device of the invention.

FIGS. 8A and 8B are diagrams each showing a panel that is one mode ofthe display device of the invention.

FIGS. 9A to 9F are views each showing an example of an electronic deviceusing a display device of the invention.

FIGS. 10A and 10B are graphs each showing temperature characteristicsand characteristics with time of a light emitting element.

FIGS. 11A and 11B are graphs showing a time-varying current of a lightemitting element and a time-varying luminance of a light emittingelement respectively.

FIG. 12 is a diagram showing a configuration of a display device of theinvention.

FIG. 13 is a diagram showing a configuration of a display device of theinvention.

FIG. 14 is a diagram showing a configuration of a display device of theinvention.

FIG. 15 is a diagram showing a configuration of a display device of theinvention.

FIG. 16 is a diagram showing a configuration of a display device of theinvention.

FIG. 17 is a diagram showing a configuration of a display device of theinvention.

FIG. 18 is a diagram showing a panel that is one mode of the displaydevice of the invention.

FIG. 19 is a diagram showing a configuration of a display device of theinvention.

FIGS. 20A and 20B are diagrams each showing a configuration of a displaydevice of the invention.

FIG. 21 is a diagram showing a configuration of a display device of theinvention.

FIG. 22 is a diagram showing a configuration of a display device of theinvention.

FIG. 23 is a diagram showing a configuration of a display device of theinvention.

FIG. 24 is a diagram showing a configuration of a display device of theinvention.

FIG. 25 is a diagram showing a configuration of a display device of theinvention.

FIG. 26 is a graph showing current density-voltage characteristics of alight emitting element.

FIG. 27 is a graph showing current density-voltage characteristics of alight emitting element.

FIG. 28 is a graph showing changes of n and s.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention will be fully described by way of EmbodimentModes and Embodiments with reference to the accompanying drawings, it isto be understood that various changes and modifications will be apparentto those skilled in the art. Therefore, unless such changes andmodifications depart from the scope of the invention hereinafterdefined, they should be construed as being included therein. Note thatin the structures of the invention described below, the same componentsare denoted by the same reference numerals in all the drawings.

Embodiment Mode 1

A display device of the invention has a light emitting element 13 and amonitoring light emitting element 66 that are both provided over asubstrate 20. The light emitting element 13 and the monitoring lightemitting element 66 are formed by the same process under the samemanufacturing conditions, and exhibit the same characteristics orsubstantially the same characteristics with respect to changes inambient temperature and changes with time.

The display device of the invention further has a constant currentsource 105 and a buffer amplifier 110. These circuits may be formed overthe same substrate 20 as the light emitting element 13 and themonitoring light emitting element 66, or may be formed on anothersubstrate.

A pixel area 40 provided over the substrate 20 includes a plurality ofpixels arranged in matrix. Each of the pixels has the light emittingelement 13 and at least two transistors. In this embodiment mode, only adriving transistor 12 connected in series to the light emitting element13 is shown. Drivers (a first gate driver 41, a second gate driver 42,and a source driver 43 are shown herein) are also provided over thesubstrate 20 and control light emission or non-light emission andluminance of each of the pixels. One of two electrodes of the lightemitting element 13 is connected to an opposite power supply 18, and theother thereof is connected to an output terminal of the buffer amplifier110 through the driving transistor 12.

One or a plurality of monitoring light emitting elements 66 are providedover the substrate 20. One of two electrodes of the monitoring lightemitting element 66 is connected to the opposite power supply 18, andthe other thereof is connected to an input terminal of the bufferamplifier 110 thorough a limiter transistor 111.

A monitoring circuit 64 including one or a plurality of monitoring lightemitting elements 66 may be provided in the pixel area 40 or the otherarea. However, the monitoring circuit 64 is preferably provided in thearea other than the pixel area 40 so as not to influence image display.

A constant current is supplied from the constant current source 105 tothe monitoring light emitting element 66. When changes in ambienttemperature and changes with time occur in this state, the resistance ofthe monitoring light emitting element 66 itself varies. Thus, since aconstant current is supplied to the monitoring light emitting element66, a potential difference between the two electrodes of the monitoringlight emitting element 66 changes.

In the case of the aforementioned structure, the potential of oneelectrode of the monitoring light emitting element 66, which isconnected to the opposite power supply 18, does not change, and thepotential of the other electrode (referred to as a first electrodeherein) of the monitoring light emitting element 66, which is connectedto the constant current source 105, changes. The changed potential ofthe first electrode of the monitoring light emitting element 66 issupplied to the buffer amplifier 110.

The potential of one electrode of the monitoring light emitting element66 is inputted to the input terminal of the buffer amplifier 110. Apotential outputted from the output terminal of the buffer amplifier 110is supplied to a first electrode of the light emitting element 13through the driving transistor 12.

In the structure shown in the drawing, a inverting input terminal andthe output terminal of the buffer amplifier 110 are connected to eachother. The input terminal of the buffer amplifier 110 is connected tothe first electrode of the monitoring light emitting element 66 whilethe output terminal of the buffer amplifier 110 is connected to thefirst electrode of the light emitting element 13.

The buffer amplifier 110 is provided in order to prevent potentialvariations. Accordingly, other circuits may be used instead of thebuffer amplifier 110 as long as they are capable of preventing potentialvariations. That is, a circuit for preventing potential variations isprovided between the monitoring light emitting element 66 and the lightemitting element 13 when the potential of one electrode of themonitoring light emitting element 66 is transmitted to the lightemitting element 13. Such a circuit is not limited to the aforementionedbuffer amplifier 110 and a circuit with any configuration may beemployed.

The aforementioned buffer amplifier 110 is a circuit for preventingpotential variations, and can be referred to as a circuit for outputtinga potential equal to an inputted potential.

A first structure of the display device of the invention ischaracterized by having the limiter transistor 111 connected in seriesto the monitoring light emitting element 66 (see FIG. 1).

A gate electrode of the limiter transistor 111 is connected to a powersupply 112. The limiter transistor 111 is on, and the power supply 112supplies a potential that turns on the limiter transistor 111. One of asource electrode and a drain electrode of the limiter transistor 111 isconnected to the first electrode of the monitoring light emittingelement 66, and the other thereof is connected to the input terminal ofthe buffer amplifier 110.

The limiter transistor 111 is provided in order to prevent an excessivecurrent from flowing through the monitoring light emitting element 66.Even when an anode and a cathode of the monitoring light emittingelement 66 are short-circuited, the limiter transistor 111 prevents themonitoring light emitting element 66 from being damaged due to anexcessive current flowing through the short-circuited portion.

In order to prevent an excessive current flowing through the monitoringlight emitting element 66, the channel length (L1) and the channel width(W1) of the driving transistor 12, and the channel length (L2) and thechannel width (W2) of the limiter transistor 111 are preferably designedso as to satisfy L1/W1:L2/W2=1:2 to 1:10.

The driving transistor 12 or the limiter transistor 111 is not onetransistor but corresponds to two transistors connected in series insome cases. In such a case, the channel length and the channel width ofthe driving transistor 12 or the limiter transistor 111 correspond tothe total channel length and the total channel width of the twotransistors connected in series, respectively.

In the structure shown in the drawing, the limiter transistor 111 is aP-channel transistor, though the invention is not limited to this and anN-channel transistor may be used as well.

In the structure shown in the drawing, the number of the monitoringlight emitting elements 66 is the same as the number of the lightemitting elements 13 of one column in the pixel area 40, though thenumber thereof is not limited to this. At least one monitoring lightemitting element 66 is only required to be provided.

A second structure of the display device of the invention is describednext with reference to FIG. 12. The second structure is characterized byhaving an AC transistor 113 connected in series to the monitoring lightemitting element 66.

A gate electrode of the AC transistor 113 is connected to the inputterminal of the buffer amplifier 110 through a switch 116. The gateelectrode of the AC transistor 113 is also connected to an AC powersupply 115 through a switch 117. One of a source electrode and a drainelectrode of the AC transistor 113 is connected to an AC power supply114, and the other thereof is connected to the first electrode of themonitoring light emitting element 66. The AC transistor 113 is providedin order to apply a reverse bias voltage to the monitoring lightemitting element 66.

When a reverse bias voltage is applied to the monitoring light emittingelement 66, the switch 116 is turned off so that the buffer amplifier110 is not electrically connected to the monitoring light emittingelement 66. In addition, the switch 117 is turned on and the potentialof the AC power supply 115 is supplied to the AC transistor 113, therebythe AC transistor 113 is turned on. Then, the relative magnitude betweenthe potential of the opposite power supply 18 and the potential of theAC power supply 114 is arbitrarily set. By applying a reverse biasvoltage to the monitoring light emitting element 66, a current islocally supplied to a short-circuited portion of the anode and thecathode of the monitoring light emitting element 66, thereby theshort-circuited portion can be insulated. Thus, it is possible tocorrect defects due to the short-circuited portion of the monitoringlight emitting element 66.

A capacitor 126 is provided in order to maintain the potential of theinput terminal of the buffer amplifier 110 when applying a reverse biasvoltage to the monitoring light emitting element 66. However, theinvention is not limited to the capacitor 126, and other circuitscapable of maintaining the potential of the input terminal of the bufferamplifier 110 may be used as well.

On the other hand, when a forward bias voltage is applied to themonitoring light emitting element 66, the switch 116 is turned on whilethe switch 117 is turned off.

In the structure shown in the drawing, the AC transistor 113 is aP-channel transistor, though the invention is not limited to this, andan N-channel transistor may also be used. Further, although the gateelectrode of the AC transistor 113 is connected to the input terminal ofthe buffer amplifier 110, the invention is not limited to this. Acontrol circuit may be provided independently to control the on/offstate of the AC transistor 113.

The aforementioned second structure can be freely combined with theaforementioned first structure.

A third structure of the display device of the invention is describednext with reference to FIG. 13. The third structure is characterized byhaving the capacitor 126 connected to the input terminal of the bufferamplifier 110, a first switch 121 provided between the first electrodeof the light emitting element 13 and the output terminal of the bufferamplifier 110, a second switch 122 provided between the first electrodeof the light emitting element 13 and an AC power supply 125, a thirdswitch 123 provided between the first electrode of the monitoring lightemitting element 66 and the input terminal of the buffer amplifier 110,a fourth switch 124 provided between the first electrode of themonitoring light emitting element 66 and the AC power supply 125, and afifth switch 128 provided between the constant current source 105 andthe input terminal of the buffer amplifier 110. For the first switch121, the second switch 122, the third switch 123, the fourth switch 124,and the fifth switch 128, a known element having a switching function,such as a transistor may be employed.

When a reverse bias voltage is applied to the light emitting element 13and the monitoring light emitting element 66, a control circuit 127brings the first switch 121, the third switch 123, and the fifth switch128 into a non-conductive state whereas the second switch 122 and thefourth switch 124 into a conductive state. Then, the relative magnitudebetween the potential of the opposite power supply 18 and the potentialof the AC power supply 125 is arbitrarily set. As set forth above, byapplying a reverse bias voltage to the light emitting element 13 and themonitoring light emitting element 66, a short-circuited portion can beinsulated and defects due to the short-circuited portion can becorrected.

On the other hand, when a forward bias voltage is applied to the lightemitting element 13 and the monitoring light emitting element 66, thecontrol circuit 127 brings the first switch 121, the third switch 123,and the fifth switch 128 into a conductive state whereas the secondswitch 122 and the fourth switch 124 into a non-conductive state.

The capacitor 126 is provided in order to maintain the potential of theinput terminal of the buffer amplifier 110 when applying a reverse biasvoltage to the light emitting element 13 and the monitoring lightemitting element 66. However, the invention is not limited to thecapacitor 126, and other circuits capable of maintaining the potentialof the input terminal of the buffer amplifier 110 may be used as well.

The aforementioned third structure can be freely combined with one orboth of the aforementioned first and second structures.

A fourth structure of the display device of the invention is describednext with reference to FIG. 14. The fourth structure is characterized byhaving a current source transistor 134 instead of the constant currentsource 105.

The current source transistor 134 is connected in series to themonitoring light emitting element 66, and a gate electrode thereof isconnected to a power supply 135. One of a source electrode and a drainelectrode of the current source transistor 134 is connected to the firstelectrode of the monitoring light emitting element 66 and the otherthereof is connected to a power supply 133.

The current source transistor 134 operates in a saturation region inorder to be used as a current source. Accordingly, the potentials of thepower supplies 133 and 135 are arbitrarily set to adjust a gate-sourcevoltage of the current source transistor 134. In order to operate thecurrent source transistor 134 in a saturation region, the ratio of thechannel length L to the channel width W (L/W) of the current sourcetransistor 134 is preferably set to 2 to 100.

In the structure shown in the drawing, the current source transistor 134is a P-channel transistor, though the invention is not limited to thisand an N-channel transistor may be used as well.

The aforementioned fourth structure can be freely combined with one ormore of the aforementioned first to third structures.

A fifth structure of the display device of the invention is describednext with reference to FIG. 15. The fifth structure is characterized byhaving a resistor 140 provided between the input terminal of the bufferamplifier 110 and the monitoring light emitting element 66. The resistor140 may be either a variable resistor or a fixed resistor.

The resistor 140 is provided in order to adjust a difference between thetotal amount of current of the monitoring light emitting element 66 andthe total amount of current of the light emitting element 13 during acertain period (e.g., during one frame period).

If the monitoring light emitting element 66 operates normally using theconstant current source 105, the duty ratio of the monitoring lightemitting element 66 is 100%. Meanwhile, the duty ratio of the lightemitting element 13 is about 70% even when a white image is displayed onthe entire screen, and it is less than 70% if the lighting ratio istaken into consideration. In other words, in normal operation, changeswith time of the monitoring light emitting element 66 progress morerapidly than changes with time of the light emitting element 13.

Therefore, according to the fifth structure, the resistor 140 isprovided to make the current value of the monitoring light emittingelement 66 lower than that of the light emitting element 13 at a certainmoment, thereby the total amount of current during a certain period ismade the same in both of the monitoring light emitting element 66 andthe light emitting element 13. As a result, changes with time progressat the same rate, and a power supply potential can be corrected moreaccurately in view of the changes with time.

The aforementioned fifth structure can be freely combined with one ormore of the aforementioned first to fourth structures.

A sixth structure of the display device of the invention is describednext with reference to FIG. 17. The sixth structure is characterized byhaving a forward bias transistor 132 connected in series to themonitoring light emitting element 66. A gate electrode of the forwardbias transistor 132 is connected to a gate line of the same row as aswitching transistor 11 included in a pixel 10. One of a sourceelectrode and a drain electrode of the forward bias transistor 132 isconnected to the first electrode of the monitoring light emittingelement 66, and the other thereof is connected to a forward bias powersupply 131. The forward bias transistor 132 is provided in order toapply a forward bias voltage to the monitoring light emitting element66.

When a forward bias voltage is applied to the monitoring light emittingelement 66, the forward bias transistor 132 is turned on, and therelative magnitude of the potential of the opposite power supply 18 andthe potential of the forward bias power supply 131 is arbitrarily set.

By applying a forward bias voltage to the monitoring light emittingelement 66, a current is locally supplied to a short-circuited portionof the monitoring light emitting element 66, thereby the short-circuitedportion can be insulated. Thus, it is possible to correct defects due tothe short-circuited portion of the monitoring light emitting element 66.

In the aforementioned structure, the limiter transistor 111 is providedin addition to the forward bias transistor 132. The aforementioned sixthstructure can be freely combined with one or more of the aforementionedfirst to fifth structures.

According to one or more of the aforementioned first to sixthstructures, a power supply potential can be corrected in accordance withchanges in ambient temperature and changes with time. In addition,according to the invention, the correction can be performed without useroperation. Thus, the correction can be continued after a device issupplied to an end user, which is expected to result in longer life ofthe device.

In the case of color display, electroluminescent layers with differentemission wavelengths may be formed in each pixel, and typically,electroluminescent layers corresponding to each color of red (R), green(G), and blue (B) are formed in each pixel. In such a case, at least themonitoring light emitting element 66, the constant current source 105,and the buffer amplifier 110 corresponding to each color of red, green,and blue are provided, and a power supply potential may be corrected inaccordance with each color.

Embodiment Mode 2

An example of a structure of the display device of the invention isdescribed with reference to drawings. The display device of theinvention has a plurality of pixels 10 each including a plurality ofelements, which are provided in an area where a source line Sx (x is anatural number, 1=x=m) and a gate line Gy (y is a natural number, 1=y=n)cross each other with an insulator interposed therebetween (see FIG.2A). The pixel 10 has the light emitting element 13, a capacitor 16, andtwo transistors. One of the two transistors is the switching transistor11 for controlling video signal input to the pixel 10, and the other isthe driving transistor 12 for controlling light emission or non-lightemission of the light emitting element 13. The switching transistor 11and the driving transistor 12 are field effect transistors, and each hasthree terminals of a gate electrode, a source electrode, and a drainelectrode.

A gate electrode of the switching transistor 11 is connected to the gateline Gy, one of a source electrode and a drain electrode thereof isconnected to the source line Sx, and the other is connected to a gateelectrode of the driving transistor 12. One of a source electrode and adrain electrode of the driving transistor 12 is connected to a powersupply line Vx (x is a natural number, 1=x=m), and the other isconnected to a pixel electrode of the light emitting element 13. Anopposite electrode of the light emitting element 13 is connected to theopposite power supply 18. The capacitor 16 is provided between the gateelectrode and the source electrode of the driving transistor 12.

The conductivity of the switching transistor 11 and the drivingtransistor 12 is not limited, and both of an N-channel transistor and aP-channel transistor may be used. In the structure shown in the drawing,the switching transistor 11 is an N-channel transistor while the drivingtransistor 12 is a P-channel transistor. The potential of the powersupply line Vx and the potential of the opposite power supply 18 are notlimited either, though different potentials are applied to the powersupply line Vx and the opposite power supply 18 so as to apply a forwardbias voltage or a reverse bias voltage to the light emitting element 13.

The display device of the invention having the aforementioned structureis characterized by having two transistors in the pixel 10. According tothe aforementioned structure, the number of transistors laid out in eachpixel 10 can be reduced. A smaller number of transistors laid out ineach pixel 10 naturally reduces the number of wirings to be disposed,leading to a high aperture ratio, high definition, and high yield. Whenthe high aperture ratio is achieved, the luminance of the light emittingelement can be reduced with the increase in light emitting area. Thatis, the current density of the light emitting element can be reduced.Accordingly, driving voltage can be reduced, which results in lowerpower consumption. In addition, the reliability of the light emittingelement 13 can be improved with a lower driving voltage.

The display device of the invention is characterized in that the drivingtransistor 12 operates in a linear region. According to this, thedriving voltage of the light emitting element 13 can be made lower thanin the case where the driving transistor operates in a saturationregion, leading to lower power consumption.

A semiconductor included in the switching transistor 11 and the drivingtransistor 12 may be formed using any of an amorphous semiconductor(amorphous silicon), a microcrystalline semiconductor, a polycrystallinesemiconductor (polysilicon), and an organic semiconductor. Themicrocrystalline semiconductor may be formed by using silane gas (SiH₄)and fluorine gas (F₂), or silane gas and hydrogen gas, or by laserirradiation after the formation of a thin film using the aforementionedgases.

The respective gate electrodes of the switching transistor 11 and thedriving transistor 12 are formed as a single layer or stacked layersusing a conductive material. For example, it is preferable to adopt astacked structure of tungsten (W) and tungsten nitride (WN), a stackedstructure of molybdenum (Mo), aluminum (Al), and molybdenum (Mo), or astacked structure of molybdenum (Mo) and molybdenum nitride (MoN).

Conductive layers (source and drain wirings) connected to impurityregions (source electrode and drain electrode) of the switchingtransistor 11 and the driving transistor 12 are formed as a single layeror stacked layers using a conductive material. For example, it ispreferable to adopt a stacked structure of titanium (Ti), aluminumsilicon (Al—Si), and titanium (Ti), a stacked structure of molybdenum(Mo), aluminum silicon (Al—Si), and molybdenum (Mo), or a stackedstructure of molybdenum nitride (MoN), aluminum silicon (Al—Si), andmolybdenum nitride (MoN). Alternatively, an aluminum-based materialcontaining nickel or an aluminum-based alloy containing nickel and oneor both of carbon and silicon may be used as well.

FIG. 3 shows a layout of the pixel 10 having the aforementionedstructure. Shown in this layout are the switching transistor 11, thedriving transistor 12, the capacitor 16, and a conductive layer 19corresponding to the pixel electrode of the light emitting element 13.FIG. 2B shows a cross sectional structure along a line A-B-C of thislayout. The switching transistor 11, the driving transistor 12, thelight emitting element 13, and the capacitor 16 are provided over thesubstrate 20 having an insulating surface such as glass and quartz.

The light emitting layer 13 has a stacked structure of the conductivelayer 19 corresponding to the pixel electrode, an electroluminescentlayer 33, and a conductive layer 34 corresponding to the oppositeelectrode. If both of the conductive layers 19 and 34 transmit light,the light emitting element 13 emits light in the directions of theconductive layer 19 and the conductive layer 34 (dual emission).Meanwhile, if one of the conductive layers 19 and 34 transmits light andthe other blocks light, the light emitting element 13 emits light onlyin the direction of the conductive layer 19 or the direction of theconductive layer 34 (top emission or bottom emission). FIG. 2B shows across sectional structure in the case where the light emitting element13 performs the bottom emission.

The capacitor 16 is provided between the gate electrode and the sourceelectrode of the driving transistor 12, and holds a gate-source voltageof the driving transistor 12. The capacitor 16 is constituted byconductive layers 22 a and 22 b (hereinafter collectively referred to asa conductive layer 22) formed on the same layer as the gate electrodesof the switching transistor 11 and the driving transistor 12, aconductive layer 26 corresponding to the source and drain wirings of thedriving transistor 12, and an insulating layer between the conductivelayer 22 and the conductive layer 26.

The capacitor 16 is also constituted by the conductive layer 26corresponding to the source and drain wirings of the driving transistor12, a conductive layer 36 formed on the same layer as the pixelelectrode of the light emitting element 13, and an insulating layerbetween the conductive layer 26 and the conductive layer 36. As shown inthe layout of FIG. 3, the conductive layer 35 is connected to theconductive layer 36.

According to the aforementioned structure, the capacitor 16 can obtaincapacitance large enough to hold the gate-source voltage of the drivingtransistor 12. The capacitor 16 is provided under a conductive layerconstituting the power supply line, therefore, decrease in apertureratio due to the capacitor 16 can be prevented. In addition, since thegate insulating film of the switching transistor 11 and the drivingtransistor 12 is not used for the capacitor 16, gate leak current can bereduced, leading to lower power consumption.

The respective thicknesses of the conductive layers 24 to 27corresponding to the source and drain wirings of the switchingtransistor 11 and the driving transistor 12 are 500 to 2000 nm, andpreferably 500 to 1300 nm. When the respective thicknesses of theconductive layers 24 to 27 increase in this manner, the influence ofvoltage drop can be suppressed since the source line Sx and the powersupply line Vx are constituted by the conductive layers 24 to 27. Notethat increased thickness of the conductive layers 24 to 27 reduceswiring resistance, while too much increased thickness of the conductivelayers 24 to 27 results in difficulty in patterning with accuracy andmaking an even surface. In other words, the respective thicknesses ofthe conductive layers 24 to 27 may be determined within theaforementioned range taking into consideration the influences of wiringresistance, difficulty in patterning, and unevenness of the surface.

The display device of the invention is also characterized by havinginsulating layers 28 and 29 (hereinafter collectively referred to as afirst insulating layer 30) covering the switching transistor 11 and thedriving transistor 12, and a second insulating layer 31 formed over thefirst insulating layer 30. The conductive layer 19 corresponding to thepixel electrode is formed over the second insulating layer 31. If thesecond insulating layer 31 is not provided, the conductive layers 24 to27 corresponding to the source and drain wirings are formed on the samelayer as the conductive layer 19, and thus, an area for forming theconductive layer 19 is limited to an area other than the conductivelayers 24 to 27. Meanwhile, when the second insulating layer 31 isprovided, an area occupied by the conductive layer 19 increases, leadingto a high aperture ratio. This structure is effective, particularly forthe top emission. The high aperture ratio increases a light emittingarea, which results in lower driving voltage and power consumption.

The first insulating layer 30 and the second insulating layer 31 aremade of an inorganic material such as silicon oxide and silicon nitride,or an organic material such as polyimide and acrylic. The firstinsulating layer 30 and the second insulating layer 31 may be made ofthe same material or different materials. As the insulating layermaterial, a siloxane-based material or a material including siloxane maybe used, which is, for example, composed of a skeleton formed by thebond of silicon (Si) and oxygen (O). The siloxane-based materialincludes an organic group containing at least hydrogen (such as an alkylgroup or aromatic hydrocarbon) as a substituent. Alternatively, a fluorogroup may be used as the substituent. Further alternatively, a fluorogroup and an organic group containing at least hydrogen may be used asthe substituent.

The second insulating layer 31 may be formed of an organic materialcapable of making some thickness to reduce the unevenness of the bottomlayer. If the second insulating layer 31 is made of an organic material,it is preferable that a third insulating layer 37 be made of nitridefunctioning as a barrier film (specifically, silicon nitride) in orderto prevent degasification.

A bank layer (also called an insulating layer) 32 may be formed ofeither an organic material or an inorganic material. However, since anelectroluminescent layer of the light emitting element 13 is provided soas to be in contact with the bank layer 32, the bank layer 32 preferablyhas a shape with a radius of curvature changing continuously such thatpinholes and the like are not formed in the electroluminescent layer. Inaddition, the bank layer 32 is preferably formed of a material thatblocks light, thereby boundaries between pixels are defined.

The display device of the invention also has the pixel area 40 where aplurality of the aforementioned pixels 10 are arranged in matrix, thefirst gate driver 41, the second gate driver 42, and the source driver43 (see FIG. 4). The first gate driver 41 and the second gate driver 42are disposed so as to face each other with the pixel area 40 interposedtherebetween, or disposed on one of the four sides of the pixel area 40.

The source driver 43 has a pulse output circuit 44, a latch 45, and aselection circuit 46. The latch 45 includes a first latch 47 and asecond latch 48. The selection circuit 46 includes a transistor 49 andan analog switch 50. The transistor 49 and the analog switch 50 areprovided for each column corresponding to the source line Sx. Theinverter 51 generates an inverted signal of a WE (Write Erase) signal,and it is not necessarily provided if an inverted signal of a WE signalis supplied externally.

A gate electrode of the transistor 49 is connected to a selection signalline 52, one of a source electrode and a drain electrode is connected tothe source line Sx, and the other is connected to a power supply 53. Theanalog switch 50 is provided between the second latch 48 and the sourceline Sx. In other words, an input node of the analog switch 50 isconnected to the second latch 48 while an output node thereof isconnected to the source line Sx. One of two control nodes of the analogswitch 50 is connected to the selection signal line 52 while the otheris connected to the selection signal line 52 through the inverter 51.The potential of the power supply 53 turns off the driving transistor 12included in the pixel 10. The potential of the power supply 53 is set toL level if the driving transistor 12 is an N-channel transistor, whereasthe potential of the power supply 53 is set to H level if the drivingtransistor 12 is a P-channel transistor.

The first gate driver 41 has a pulse output circuit 54 and a selectioncircuit 55. The second gate driver 42 has a pulse output circuit 56 anda selection circuit 57. The selection circuits 55 and 57 are connectedto the respective selection signal lines 52, though the selectioncircuit 57 included in the second gate driver 42 is connected to theselection signal line 52 through an inverter 58. That is, inverted WEsignals are inputted to the selection circuits 55 and 57 through therespective selection signal lines 52.

Each of the selection circuits 55 and 57 has a tri-state buffer. Aninput node of the tri-state buffer is connected to the pulse outputcircuit 54 or the pulse output circuit 56, and a control node thereof isconnected to the selection signal line 52. An output node of thetri-state buffer is connected to the gate line Gy. The tri-state bufferis brought into an operating state when a signal transmitted from theselection signal line 52 is H level while into a floating state when asignal transmitted from the selection signal line 52 is L level.

The pulse output circuit 44 included in the source driver 43, the pulseoutput circuit 54 in the first gate driver 41, and the pulse outputcircuit 56 in the second gate driver 42 correspond to a shift registerconstituted by a plurality of flip flop circuits or a decoder circuit.If a decoder circuit is used as the pulse output circuits 44, 54, and56, the source line Sx or the gate line Gy can be selected at random.When the source line Sx or the gate line Gy can be selected at random,it is possible to suppress pseudo contour generated in the case ofadopting a time gray scale method.

The configuration of the source driver 43 is not limited to theforegoing, and a level shifter and a buffer may be providedadditionally. The configurations of the first gate driver 41 and thesecond gate driver 42 are also not limited to the foregoing, and a levelshifter or a buffer may be provided additionally. Moreover, the sourcedriver 43, the first gate driver 41, and the second gate driver 42 mayinclude a protection circuit.

The display device of the invention is also characterized by having apower supply control circuit 63. The power supply control circuit 63 hasa power supply circuit 61 for supplying power to the light emittingelement 13, and a control circuit 62. The power supply circuit 61 isconnected to the pixel electrode of the light emitting element 13through the driving transistor 12 and the power supply line Vx. Theopposite power supply 18 included in the power supply circuit 61 isconnected to the opposite electrode of the light emitting element 13through the power supply line Vx.

When a forward bias voltage is applied to the light emitting element 13to supply a current thereto and emit light, the potential differencebetween the power supply line Vx and the opposite power supply 18 is setsuch that the potential of the power supply line Vx is higher than thatof the opposite power supply 18. Meanwhile, when a reverse bias voltageis applied to the light emitting element 13, the potential differencebetween the power supply line Vx and the opposite power supply 18 is setsuch that the potential of the power supply line Vx is lower than thatof the opposite power supply 18. Such a power supply setting is made bysupplying a predetermined signal from the control circuit 62 to thepower supply circuit 61.

According to the invention, a reverse bias voltage is applied to thelight emitting element 13 by using the power supply control circuit 63,thereby degradation with time of the light emitting element 13 can besuppressed and reliability can be improved. In the light emittingelement 13, an initial defect where an anode and a cathode areshort-circuited may occur due to the deposition of foreign material,pinholes due to a slight unevenness of the anode or the cathode, andunevenness of the electroluminescent layer. In a pixel having such aninitial defect, problems occur such that light emission and non-lightemission are not carried out in accordance with signals, and thus almostall currents flow through the short-circuited portion and the wholeelement emits no light, or certain pixels do not correctly emit light orno light, leading to faulty display of images. However, since a reversebias voltage can be applied to the light emitting element according tothe invention, a current is locally supplied only to a short-circuitedportion between the anode and the cathode, and the short-circuitedportion generates heat. As a result, the short-circuited portion can beoxidized or carbonized to be insulated. Thus, even when an initialdefect occurs, the defect can be corrected and images can be displayedwith high quality. Note that such insulation of the initial defect ispreferably performed before shipment of the display device. In additionto the initial defect, a defect where the anode and the cathode areshort-circuited may occur as time passes. Such a defect is also called aprogressive defect. However, according to the invention, a reverse biasvoltage can be applied to the light emitting element periodically.Therefore, even when a progressive defect occurs, the defect can becorrected and images can be displayed with high quality. Note that areverse bias voltage can be applied to the light emitting element 13 atany timing.

As set forth above, the display device of the invention is alsocharacterized by having the monitoring circuit 64 including themonitoring light emitting element 66, and a monitor control circuit 65including the constant current source, the buffer amplifier and thelike. The specific configurations of the monitoring circuit 64 and themonitor control circuit 65 are described in Embodiment Mode 1,therefore, the description thereof is omitted herein. According to theinvention having the aforementioned structures, variations in currentvalues of the light emitting element due to changes in ambienttemperature and changes with time can be suppressed, leading to improvedreliability.

Note that in the structures shown in FIGS. 1 and 17, the monitor controlcircuit 65 includes the constant current source 105 and the bufferamplifier 110. In the structure shown in FIG. 12, the monitor controlcircuit 65 includes the constant current source 105, the bufferamplifier 110, the switches 116 and 117, and the capacitor 126. In thestructure shown in FIG. 13, the monitor control circuit 65 includes theconstant current source 105, the buffer amplifier 110, the first switch121, the second switch 122, the third switch 123, the fourth switch 124,the capacitor 126, the control circuit 127, and the fifth switch 128. Inthe structure shown in FIG. 14, the monitor control circuit 65 includesthe buffer amplifier 110. In the structure shown in FIG. 15, the monitorcontrol circuit 65 includes the constant current source 105, the bufferamplifier 110, and the resistor 140.

Operation of the display device of the invention having theaforementioned structures is described next with reference to drawings.First, operation of the source driver is described with reference toFIG. 5A. A clock signal (hereinafter referred to as SCK), a clockinverted signal (hereinafter referred to as SCKB), and a start pulse(hereinafter referred as SSP) are inputted to the pulse output circuit44, and a sampling pulse is outputted to the first latch 47 at thetiming of these signals. The first latch 47 to which data is inputtedholds video signals of the first to last columns at the timing of theinputted sampling pulse. When a latch pulse is inputted to the secondlatch 48, the video signals held in the first latch 47 aresimultaneously transmitted to the second latch 48.

When it is assumed that an L level WE signal is transmitted from theselection signal line 52 during a period T1 while an H level WE signalis transmitted during a period T2, the selection circuit 46 operatesduring each period in the following manner. Each of the periods T1 andT2 corresponds to half of a horizontal scan period, and the period T1 iscalled a first subgate selection period whereas the period T2 is calleda second subgate selection period.

During the period T1 (first subgate selection period), an L level WEsignal is transmitted from the selection signal line 52, the transistor49 is turned on, and the analog switch 50 is brought into anon-conductive state. Then, the plurality of signal lines S1 to Sn areelectrically connected to the power supply 53 through the transistor 49provided in each column. That is, the potentials of the signal lines S1to Sn become equal to the potential of the power supply 53.

At this time, the switching transistor 11 included in the pixel 10 ison, and the potential of the power supply 53 is transmitted to the gateelectrode of the driving transistor 12 through the switching transistor11. Thus, the driving transistor 12 is turned off and the two electrodesof the light emitting element 13 have the same potential. That is, nocurrent flows through the two electrodes of the light emitting element13, thereby no light is emitted. In this manner, the potential of thepower supply 53 is transmitted to the gate electrode of the drivingtransistor 12 regardless of the state of a video signal inputted to avideo line, and thus the driving transistor 12 is turned off and the twoelectrodes of the light emitting element 13 have the same potential.Such operation is called erasing operation.

During the period T2 (second subgate selection period), an H level WEsignal is transmitted from the selection signal line 52, the transistor49 is turned off, and the analog switch 50 is brought into a conductivestate. Then, the video signals held in the second latch 48 aresimultaneously transmitted to the signal lines S1 to Sn for one row. Atthis time, the switching transistor 11 included in the pixel 10 is on,and the video signal is transmitted to the gate electrode of the drivingtransistor 12 through the switching transistor 11. Thus, the drivingtransistor 12 is turned on or off depending on the inputted videosignal, thereby the two electrodes of the light emitting element 13 havedifferent potentials or the same potential. More specifically, when thedriving transistor 12 is turned on, the two electrodes of the lightemitting element 13 have different potentials and a current flowstherethrough, namely, the light emitting element 13 emits light. Notethat the same current flows through the light emitting element 13 andbetween the source and the drain of the driving transistor 12.

On the other hand, when the driving transistor 12 is turned off, the twoelectrodes of the light emitting element 13 have the same potential andno current flows therethrough, namely, the light emitting element 13emits no light. In this manner, the driving transistor 12 is turned onor off depending on a video signal, and the two electrodes of the lightemitting element 13 have different potentials or the same potential.Such operation is called writing operation.

Operation of the first gate driver 41 and the second gate driver 42 isdescribed next. A clock signal (G1CK), a clock inverted signal (G1CKB),and a start pulse (G1SP) are inputted to the pulse output circuit 54,and pulses are sequentially outputted to the selection circuit 55 at thetiming of these signals. A clock signal (G2CK), a clock inverted signal(G2CKB), and a start pulse (G2SP) are inputted to the pulse outputcircuit 56, and pulses are sequentially outputted to the selectioncircuit 57 at the timing of these signals. FIG. 5B shows the potentialsof pulses supplied to the selection circuits 55 and 57 of the i-th,j-th, k-th, and p-th rows (i, j, k, and p are natural numbers, 1=i, j,k, p=n).

When it is assumed that an L level WE signal is transmitted from theselection signal line 52 during a period T1 while an H level WE signalis transmitted during a period T2 similarly to the operation of thesource driver 43, the selection circuit 55 in the first gate driver 41and the selection circuit 57 in the second gate driver 42 operate ineach period in the following manner. In the timing chart of FIG. 5B, thepotential of the gate line Gy (y is a natural number, 1=y=n) thatreceives a signal from the first gate driver 41 is denoted by Gy41,while the potential of the gate line that receives a signal from thesecond gate driver 42 is denoted by Gy42. It is clear that Gy41 and Gy42denote the same wiring.

In the period T1 (first subgate selection period), an L level WE signalis transmitted from the selection signal line 52. Thus, an L level WEsignal is inputted to the selection circuit 55 in the first gate driver41, thereby the selection circuit 55 is brought into a floating state.On the other hand, an inverted WE signal, namely an H level WE signal isinputted to the selection circuit 57 in the second gate driver 42,thereby the selection circuit 57 is brought into an operating state.That is, the selection circuit 57 transmits an H level signal (rowselection signal) to a gate line Gi of the i-th row such that the gateline Gi has the same potential as the H level signal. In other words,the gate line Gi of the i-th row is selected by the second gate driver42.

As a result, the switching transistor 11 included in the pixel 10 isturned on. Then, the potential of the power supply 53 included in thesource driver 43 is transmitted to the gate electrode of the drivingtransistor 12, thereby the driving transistor 12 is turned off and thetwo electrodes of the light emitting element 13 have the same potential.That is, the erasing operation where the light emitting element 13 emitsno light is performed in this period.

In the period T2 (second subgate selection period), an H level WE signalis transmitted from the selection signal line 52. Thus, an H level WEsignal is inputted to the selection circuit 55 in the first gate driver41, thereby the selection circuit 55 is brought into an operating state.That is, the selection circuit 55 transmits an H level signal to thegate line Gi of the i-th row such that the gate line Gi has the samepotential as the H level signal. In other words, the gate line Gi of thei-th row is selected by the first gate driver 41.

As a result, the switching transistor 11 included in the pixel 10 isturned on. Then, the video signal is transmitted from the second latch48 in the source driver 43 to the gate electrode of the drivingtransistor 12, thereby the driving transistor 12 is turned on or off andthe two electrodes of the light emitting element 13 have differentpotentials or the same potential. That is, the writing operation wherethe light emitting element 13 emits light or no light is performed inthis period. Meanwhile, an L level signal is inputted to the selectioncircuit 57 in the second gate driver 42, and the selection circuit 57 isbrought into a floating state.

As set forth above, the gate line Gy is selected by the second gatedriver 42 during the period T1 (first subgate selection period) whileselected by the first gate driver 41 during the period T12 (secondsubgate selection period). That is, the gate line is controlled by thefirst gate driver 41 and the second gate driver 42 in a complementarymanner. The erasing operation is performed during one of the first andsecond subgate selection periods, and the writing operation is performedduring the other thereof.

During a period when the first gate driver 41 selects the gate line Giof the i-th row, the second gate driver 42 does not operate (selectioncircuit 57 is in a floating state), or transmits a row selection signalto the gate lines other than the i-th row. Similarly, during a periodwhen the second gate driver 42 transmits a row selection signal to thegate line Gi of the i-th row, the first gate driver 41 is in a floatingstate, or transmits a row selection signal to the gate lines other thanthe i-th row.

According to the invention performing the aforementioned operation, thelight emitting element 13 can be turned off forcibly, leading to anincreased duty ratio. Further, the light emitting element 13 can beturned off forcibly without providing a TFT for discharging the chargesof the capacitor 16, which results in a high aperture ratio. When thehigh aperture ratio is achieved, the luminance of the light emittingelement can be reduced with the increase in light emitting area. Thatis, the driving voltage can be reduced and thus power consumption can bereduced.

The invention is not limited to the aforementioned embodiment mode wherea gate selection period is divided into two periods. The gate selectionperiod may be divided into three or more periods.

Embodiment Mode 3

Description is made on an example of a pixel circuit that can be appliedto the display device of the invention. FIG. 6A shows a pixel circuitwhere an erasing transistor 91 and an erasing gate line Ry are added tothe pixel 10 shown in FIG. 2A (one pixel includes three TFTs). Theerasing transistor 91 can forcibly stop current flow in the lightemitting element 13. Therefore, a lighting period can start with orimmediately after the start of a writing period without waiting forsignals to be written to all the pixels 10. Accordingly, duty ratio canbe increased, and particularly moving images can be displayed with highquality.

FIG. 6B shows a pixel circuit where the driving transistor 12 in thepixel 10 shown in FIG. 6A is omitted and transistors 92 and 93 and apower supply line Vax (x is a natural number, 1=x=m) are additionallyprovided (one pixel includes four TFTs). The power supply line Vax isconnected to a power supply 94. According to this structure, a gateelectrode of the transistor 92 is connected to the power supply line Vaxwith a constant potential, thereby the potential of the gate electrodeof the transistor 92 is fixed and the transistor 92 operates in asaturation region. Meanwhile, the transistor 93 operates in a linearregion, and a video signal including data on light emission or non-lightemission of the pixel 10 is inputted to a gate electrode of thetransistor 93. Since the transistor 93 operating in a linear region hasa small source-drain voltage, a slight change in the gate-source voltageof the transistor 93 does not influence a current value flowing throughthe light emitting element 13. Thus, the current value flowing throughthe light emitting element 13 is determined by the transistor 92operating in a saturation region. According to the invention having theaforementioned structure, it is possible to suppress luminanceunevenness due to variations in characteristics of the transistor 92 andincrease the image quality.

As another pixel circuit, a pixel circuit where the switching transistor11 in the pixel 10 shown in FIG. 2A is omitted (one pixel includes oneTFT) may be adopted as well. In this case, operation is performedsimilarly to that of a passive matrix display.

Alternatively, a pixel circuit using a current mirror circuit may alsobe adopted.

Either an analog video signal or a digital video signal may be used inthe display device of the invention. If a digital video signal is used,the video signal may be either a voltage or a current. That is, a videosignal inputted to a pixel in light emission of a light emitting elementmay be either a constant voltage or a constant current. When a videosignal is a constant voltage, a constant voltage is applied to a lightemitting element or a constant current flows in the light emittingelement. When a video signal is a constant current, a constant voltageis applied to a light emitting element or a constant current flowsthrough the light emitting element. When a constant voltage is appliedto a light emitting element, a constant voltage drive is performed.Meanwhile, when a constant current flows through a light emittingelement, a constant current drive is performed. According to theconstant current drive, a constant current flows regardless of changesin resistance of the light emitting element. The display device of theinvention may adopt either the constant voltage drive or the constantcurrent drive, though a voltage video signal is preferably used in thedisplay device of the invention.

An electroluminescent layer is made of a material emitting light from asinglet excited state (hereinafter referred to as a singlet excitedlight emitting material) or a material emitting light from a tripletexcited state (hereinafter referred to as a triplet excited lightemitting material). For example, among light emitting elements that emitred, green, and blue light, a red light emitting element whose luminanceis reduced by half in a relatively short time is made of a tripletexcited light emitting material and the rest are made of a singletexcited light emitting material. A triplet excited light emittingmaterial has the advantage that the material has a good luminousefficiency and consumes less power to obtain the same luminance.

Alternatively, a red light emitting element and a green light emittingelement may be made of a triplet excited light emitting material and ablue light emitting element may be made of a singlet excited lightemitting material. Low power consumption can be further achieved when agreen light emitting element having high visibility is made of a tripletexcited light emitting material. As an example of a triplet excitedlight emitting material, there are a metal complex used as a dopant, ametal complex having platinum that is a third transition series elementas a central metal, a metal complex having indium as a central metal,and the like. Further, the electroluminescent layer may be formed of anyof a low molecular weight material, a medium molecular weight material,and a high molecular weight material.

The light emitting element may adopt a forward stacked structure wherean anode, an electroluminescent layer, and a cathode are stacked in thisorder, or a reverse stacked structure where a cathode, anelectroluminescent layer, and an anode are stacked in this order. Theanode or the cathode of the light emitting element may be made of indiumtin oxide (ITO) that transmits light, ITO added with silicon oxide,indium zinc oxide (IZO), zinc oxide doped with gallium (Ga) (GZO), andthe like.

The light emitting element may also adopt a structure where a pluralityof electroluminescent layers and charge generation layers are stackedbetween an anode and a cathode such that an anode, an electroluminescentlayer, a charge generation layer, . . . , an electroluminescent layer, acharge generation layer, . . . , an electroluminescent layer, and acathode are stacked in this order. Such an element is also called atandem element. The charge generation layer is made of an inorganicsemiconductor such as metal or molybdenum oxide, an organic compounddoped with lithium, or the like.

When color display is performed using a panel having a light emittingelement, electroluminescent layers having different emission wavelengthbands may be provided in each pixel. Typically, an electroluminescentlayer corresponding to each color of red (R), green (G), and blue (B) isprovided. In such a case, the monitoring light emitting element 66corresponding to each color of red, green, and blue may be provided tocorrect a power supply potential for each color. At this time, colorpurity can be increased and a pixel portion can be prevented from havinga mirror surface (glare) by providing a filter (colored layer) thattransmits light of a specific wavelength band on the light emitting sideof a light emitting element. Providing a filter (colored layer) can omita circular polarizer or the like that is conventionally required and caneliminate the loss of light emitted from the electroluminescent layer.Further, change in hue that occurs when a pixel area is obliquely seencan be reduced.

The electroluminescent layer can have a structure that emits monochromeor white light. If a white light emitting material is used, a filterthat transmits light having a specific wavelength is provided on thelight emitting side of a light emitting element, thereby color displaycan be performed.

Embodiment Mode 4

Changes with time of a light emitting element progress rapidly in theinitial stages and gradually slow down with time. Accordingly, in adisplay device using a light emitting element, it is preferable toperform an initial aging process where initial changes with time occurin all the light emitting elements before adjustment of the luminance ofthe light emitting elements (e.g., before shipment of the displaydevice).

When initial drastic changes with time of a light emitting element occurin advance by such an initial aging process, changes with time do notprogress rapidly thereafter, which reduces the phenomenon due to thechanges with time such as image burn-in.

The initial aging process is performed by activating a light emittingelement only during a certain period, and preferably by applying avoltage higher than usual. According to this, initial changes with timeoccur in a short time and the initial aging process can be completedimmediately.

If the display device of the invention operates using a rechargeablebattery, it is preferable to perform, during charging the display devicethat is not in use, a process of lighting or flashing all the pixels, aprocess of displaying an image whose contrast is inverted relative to anormal image (e.g., standby display or the like), a process of detectinga pixel that emits light at a low frequency by sampling a video signaland lighting or flashing the pixel, and the like. The aforementionedprocess is performed in order to reduce image burn-in during a periodwhen the display device is not in use, and called a flashout process.Even when image burn-in occurs after the flashout process, thedifference between the brightest point and the darkest point of theburned-in image can be set to five level gray scale or less, and morepreferably one level gray scale or less. In order to reduce imageburn-in, a fixed image may be reduced as much as possible in addition tothe aforementioned process.

Embodiment Mode 5

Description is made on a panel that is one mode of the display device ofthe invention, which incorporates the pixel area 40, the first gatedriver 41, the second gate driver 42, and the source driver 43. Thepixel area 40 having a plurality of pixels each including the lightemitting element 13, the first gate driver 41, the second gate driver42, the source driver 43, and a connection film 407 are provided overthe substrate 20 (see FIG. 7A). The connection film 407 is connected toan external circuit (IC chip).

FIG. 7B is a cross sectional view along a line A-B of the panel. FIG. 7Bshows the driving transistor 12, the light emitting element 13, and thecapacitor 16 that are formed in the pixel area 40, and a CMOS circuit410 formed in the source driver 43.

A sealing material 408 is provided at the periphery of the pixel area40, the first gate driver 41, the second gate driver 42, and the sourcedriver 43. The light emitting element 13 is sealed with the sealingmaterial 408 and an opposite substrate 406. This sealing process isperformed in order to protect the light emitting element 13 frommoisture. In this embodiment mode, a cover material (made of glass,ceramics, plastic, metal or the like) is used for sealing, though a heatcurable resin or a UV light curable resin may be used as well as a highbarrier thin film such as metal oxide and nitride. The elements over thesubstrate 20 are preferably formed of a crystalline semiconductor(polycrystalline silicon) having superior characteristics in mobilityand the like as compared with an amorphous semiconductor, thereby theelements can be monolithically formed on the same surface. The panelhaving the aforementioned structure can reduce the number of externalICs to be connected, leading to reduction in size, weight, andthickness.

FIG. 18 is a cross sectional view along a line C-D of the panel, andshows the driving transistor 12, the light emitting element 13, and thecapacitor 16 that are provided in the pixel area 40, a CMOS circuit 412provided in the first gate driver 41, and a CMOS circuit 411 provided inthe second gate driver 42. The panel shown in the drawing ischaracterized in that the sealing material 408 is provided so as tooverlap the first gate driver 41 and the second gate driver 42. Thisstructure achieves a narrower frame.

In the aforementioned structures shown in FIGS. 7A and 7B and FIG. 18,the pixel electrode of the light emitting element 13 transmits lightwhile the opposite electrode of the light emitting element 13 blockslight. Therefore, the light emitting element 13 performs the bottomemission.

As a structure different from the aforementioned, there is a structurewhere the pixel electrode of the light emitting element 13 blocks lightwhile the opposite electrode of the light emitting element 13 transmitslight (see FIG. 8A). In this case, the light emitting element 13performs the top emission.

As a structure different from the aforementioned, there is a structurewhere both the pixel electrode and the opposite electrode of the lightemitting element 13 transmit light (see FIG. 8B). In this case, thelight emitting element 13 performs the dual emission.

In the case of the bottom emission and the dual emission, a conductivelayer (source and drain wirings) connected to the impurity region of thedriving transistor 12 is preferably formed of aluminum (Al) combinedwith a low reflective material such as molybdenum (Mo). Specifically, astacked structure of Mo, Al—Si, and Mo, or a stacked structure of MoN,Al—Si, and MoN, or the like may be adopted. As a result, light emittedfrom the light emitting element can be prevented from reflecting thesource and drain wirings, thereby the light can be extracted outside.The display device of the invention may adopt any of the bottomemission, the top emission, and the dual emission.

The pixel area 40 may be constituted by a TFT that is formed over aninsulating surface and has a channel portion formed of an amorphoussemiconductor (amorphous silicon), and the first gate driver 41, thesecond gate driver 42, and the source driver 43 may be constituted by anIC chip. The IC chip may be attached onto the substrate 20 by COG orattached to the connection film 407 connected to the substrate 20. Theamorphous semiconductor can be easily formed over a large substrate byCVD and requires no crystallization step, and thus provides aninexpensive panel. Further, when a conductive layer is formed by dropletdischarging typified by ink jet method, a more inexpensive panel can beachieved.

Embodiment Mode 6

An electronic device provided with a pixel area including a lightemitting element includes a television set (also called a television ora television receiver), a digital camera, a digital video camera, amobile phone set (also called a mobile phone or a cellular phone), aportable information terminal such as a PDA, a portable game machine, amonitor for a computer, a computer, an audio reproducing device such asan in-car audio system, an image reproducing device provided with arecording medium such as a home game machine, and the like. Specificexamples of them are described with reference to FIGS. 9A to 9F.

A portable information terminal includes a main body 9201, a displayportion 9202 and the like (see FIG. 9A). The display devices shown inEmbodiment Modes 1 to 5 can be applied to the display portion 9202.According to the invention, a power supply potential supplied to a lightemitting element is corrected using a monitoring light emitting element,and it is thus possible to provide a display device where the influenceof variations in current of the light emitting element due to changes inambient temperature and changes with time can be suppressed.

A digital video camera includes a display portion 9701, a displayportion 9702 and the like (see FIG. 9B). The display devices shown inEmbodiment Modes 1 to 5 can be applied to the display portion 9701.According to the invention, a power supply potential supplied to a lightemitting element is corrected using a monitoring light emitting element,and it is thus possible to provide a display device where the influenceof variations in current of the light emitting element due to changes inambient temperature and changes with time can be suppressed.

A portable terminal includes a main body 9101, a display portion 9102and the like (see FIG. 9C). The display devices shown in EmbodimentModes 1 to 5 can be applied to the display portion 9102. According tothe invention, a power supply potential supplied to a light emittingelement is corrected using a monitoring light emitting element, and itis thus possible to provide a display device where the influence ofvariations in current of the light emitting element due to changes inambient temperature and changes with time can be suppressed.

A portable television set includes a main body 9301, a display portion9302 and the like (see FIG. 9D). The display devices shown in EmbodimentModes 1 to 5 can be applied to the display portion 9302. According tothe invention, a power supply potential supplied to a light emittingelement is corrected using a monitoring light emitting element, and itis thus possible to provide a display device where the influence ofvariations in current of the light emitting element due to changes inambient temperature and changes with time can be suppressed. Such atelevision set can be widely applied to a small size one incorporated ina portable terminal such as a mobile phone, a medium size one that isportable, and a large size one (e.g., 40 inches in size or more).

A portable computer includes a main body 9401, a display portion 9402and the like (see FIG. 9E). The display devices shown in EmbodimentModes 1 to 5 can be applied to the display portion 9402. According tothe invention, a power supply potential supplied to a light emittingelement is corrected using a monitoring light emitting element, and itis thus possible to provide a display device where the influence ofvariations in current of the light emitting element due to changes inambient temperature and changes with time can be suppressed.

A television set includes a main body 9501, a display portion 9502 andthe like (see FIG. 9F). The display devices shown in Embodiment Modes 1to 5 can be applied to the display portion 9502. According to theinvention, a power supply potential supplied to a light emitting elementis corrected using a monitoring light emitting element, and it is thuspossible to provide a display device where the influence of variationsin current of the light emitting element due to changes in ambienttemperature and changes with time can be suppressed.

If the aforementioned electronic devices use a rechargeable battery, thelife of them increases with reduction in power consumption, thereby thecharge of the rechargeable battery can be saved.

Embodiment Mode 7

A cross sectional structure of a display device performing color displayis described with reference to drawings. More specifically, a crosssectional structure of a display device using a white light emittingelement that emits white light and a colored layer is described. Shownbelow is a cross sectional structure of three pixels adjacent to eachother.

The driving transistor 12, the light emitting element 13 that emitswhite light (hereinafter referred to as the white light emitting element13), and the capacitor 16 are provided over the substrate 20 (see FIG.22). The white light emitting element 13 performs the top emission. Thebank layer 32 that blocks light is also provided. The bank layer 32 thatblocks light is formed by agitating carbon particles, metal particles, apigment, a colorant and the like, filtering them if necessary, and thenspin coating. Note that if carbon particles or metal particles are addedto an organic material, a surface active agent or a dispersing agent maybe added so that they are mixed evenly.

Colored layers 711 to 713 are provided over the opposite substrate 406,and typically correspond to red, green, or blue.

The white light emitting element 13 is provided so as to overlap thecolored layers 711 to 713 when the substrate 20 is attached to theopposite substrate 406. More specifically, the white light emittingelement 13 is provided such that the light emitting side thereof facesthe colored layers 711 to 713. According to the aforementionedstructure, white light emitted from the white light emitting element 13changes into red, green, or blue, thereby a color display device can beobtained.

A structure of the display device where the white light emitting element13 performs the bottom emission is described with reference to FIG. 23.In this case, as shown in the drawing, colored layers 714 to 716 areprovided as interlayer insulating layers. Alternatively, the coloredlayers 714 to 716 may be provided between the substrate 20 and thedriving transistor 12 or the capacitor 16. In such a case, an activelayer of the driving transistor 12 is preferably formed of an amorphoussemiconductor instead of a polycrystalline semiconductor in view of poorheat resistance of the colored layers. As a result, the colored layers714 to 716 can be prevented from being damaged due to heat treatmentduring manufacturing steps of a polycrystalline semiconductor.

A structure of the display device where the white light emitting element13 performs the dual emission is described with reference to FIG. 24.The structure shown in FIG. 24 is a combination of the structures shownin FIGS. 22 and 23. The colored layers 711 to 716 are provided so as tosandwich the white light emitting element 13, such that the white lightemitting element 13 emits light in the opposite directions.

An electroluminescent layer of a light emitting element is formed byvapor deposition, spin coating, ink jet method and the like. However,electroluminescent layers with different wavelength bands cannot beformed with accuracy by the aforementioned methods. Thus, in such acase, it is necessary to increase the distance between different pixelsand the distance between banks. Meanwhile, if a white light emittingelement is used as shown in the aforementioned structure, differentelectroluminescent layers are not to be formed, which is advantageous inthat the distance between pixels and the distance between banks are notrequired to increase and high definition can be achieved. In addition,since a color filter is used in a liquid crystal display device, thetechnology of the liquid crystal display device can be utilized withoutdeveloping new technology.

The width of the bank layer 32 between pixels in the lateral directionis only required to be wide enough to cover a wiring formed thereunder,specifically 7.5 to 27.5 μm, and more preferably 10 to 25 μm (see FIG.25). The narrower width of the bank layer 32 allows a higher apertureratio. The high aperture ratio increases a light emitting area, whichresults in lower driving voltage and power consumption.

Light emitted from the white light emitting element may be reflectedrepeatedly in the pixel electrode and the opposite electrode and escapeto the adjacent pixels depending on the light exit angle. Further, in adisplay area including a plurality of pixels arranged in matrix, glaredue to the pixel electrode and the opposite electrode may occur in apixel displaying black. In order to prevent such glare, an optical filmis attached in some cases, though it costs a lot.

However, according to the aforementioned structure, the bank layer 32 ismade of a material that blocks light. If the bank layer 32 blocks light,it absorbs unnecessary light to clearly define boundaries betweenpixels, and thus high definition images can be displayed. In addition,an insulating film that blocks light reduces the reflection of incidentlight, and glare can be prevented. Therefore, an optical film such as apolarizer is not necessary any more, leading to reduction in size,thickness, and weight.

A triplet excited light emitting material including a metal complex orthe like as well as a singlet excited light emitting material may beused for the electroluminescent layer of the white light emittingelement 13. As an example of a triplet excited light emitting material,there are known a metal complex used as a dopant, a metal complex havingplatinum that is a third transition series element as a central metal, ametal complex having iridium as a central metal, and the like. Thetriplet excited light emitting material is not limited to thesecompounds and it is also possible to use a compound having theaforementioned structure and having an element belonging to Groups 8 to10 of the periodic table as a central metal.

A light emitting element that emits white light may be constituted bytwo or three light emitting layers including a blue electroluminescentlayer. Alternatively, a white light emitting element may be formed byarbitrarily stacking a functional layer such as a holeinjecting/transporting layer, a hole transporting layer, an electroninjecting/transporting layer, an electron transporting layer, a lightemitting layer, an electron blocking layer, and a hole blocking layer.Further, a mixed layer or a mixed connection of these layers may also beformed. This embodiment mode can be freely combined with theaforementioned embodiment modes.

Embodiment Mode 8

The invention provides a display device having a white light emittingelement and a monitoring white light emitting element, where the dutyratio of the white light emitting element is 45 to 80% while the dutyratio of the monitoring white light emitting element is 45 to 100%.

The duty ratio of the white light emitting element is the average dutyratio of all the white light emitting elements provided in a pixel area.The duty ratio is the ratio of a lighting period to a lighting periodand a non-light emitting period such as a writing period when all theinputted video signals display white.

The invention provides a display device where the total amount ofcurrent in a white light emitting element during a certain period isless than the total amount of current in a monitoring white lightemitting element.

In this manner, the load on the white light emitting element is madedifferent from the load on the monitoring white light emitting element,and luminance decay is taken into consideration based on the amount ofcharge flowing through the white light emitting element. Accordingly,constant luminance drive can be performed where the amount of charge inthe white light emitting element is compared with the amount of chargein the monitoring white light emitting element, and the luminance of thewhite light emitting element is corrected so as to be constant.

The principle of constant luminance drive (hereinafter referred to as CLdrive) according to the invention is described below. A light emittingelement used for the description has a structure where a thin filmcontaining an organic material generating EL is sandwiched between apair of electrodes.

A current flowing through a thin film containing an organic materialgenerating EL (hereinafter also referred to as an organic thin film) iscalled a trap charge limited current (TCLC) and represented by thefollowing formula where J is a current density, V is a voltage, S is avalue determined by the material and structure of the light emittingelement, and n is a value of 2 or more.J=S·V ^(n)  (1)

The following formula can be obtained by modifying the formula (1).log J=n·log V+log S  (2)The formula (2) represents current-voltage characteristics indicated ona logarithmic scale, which is represented by a straight line with a slopof n. The smaller the value of log S becomes, the higher voltage sidethe straight line is shifted to.

FIG. 26 is a graph showing typical current density-voltagecharacteristics of a light emitting element. The element has a stackedstructure of an anode, DNTPD, NPB, Alq:C6, Alq, CaF₂, and Al. The graphshows characteristics in the initial state, characteristics after beingheld for 1000 hours at room temperature, and characteristics after beingdriven with constant current for 1000 hours at room temperature.

As shown in FIG. 26, the current density-voltage characteristics of thelight emitting element that has been driven with constant current for1000 hours at room temperature are shifted to a higher voltage side thanthe initial characteristics. Similarly, the current density-voltagecharacteristics of the light emitting element that has been held for1000 hours at room temperature without flowing current are shifted to ahigher voltage side.

FIG. 27 is a double logarithmic graph obtained by plotting theaforementioned three types of current density-voltage characteristicsbased on the formula (2) in a current density region where practicalluminance can be obtained. In the graph of FIG. 27, the currentdensity-voltage characteristics are plotted against a current density of1 to 100 mA/cm² where a luminance of 100 to 10000 cd/m² can be obtained.In the graph of FIG. 27, the current density-voltage characteristics arerepresented by straight lines with a slop of n.

FIG. 28 shows changes in n and S obtained by the graph of FIG. 27. Thegraph of FIG. 28 indicates characteristic changes in parameters of n andS based on the formula (2). The value of S does not vary when the lightemitting element is held at room temperature, and decreases drasticallywhen a current is supplied to the light emitting element. On the otherhand, the value of n decreases not only when a current is supplied tothe light emitting element but also when the light emitting element isheld for the same hours at room temperature. The rate of the decreasewhen a current is supplied to the light emitting element issubstantially the same as that when no current is supplied thereto. Thatis, n is a parameter that decreases almost exclusively with timeregardless of whether a current is supplied or not.

The result shows that n can be represented by the following formula (3)as a function of time.n=f(t)  (3)The value of n indicating a precipitous change in diode characteristicsshows that diode characteristics of the light emitting element change(the value of n decreases and the slope descends) with time regardlessof whether a current is supplied or not.

On the other hand, S is a parameter that hardly changes when the lightemitting element is held at room temperature and changes when a currentis supplied thereto. The value of S that is independent of time andvaries with current can be represented as a function of the total amountof charge Q (current×time), and the following formula can be obtained.S=g(Q)  (4)Since the value of S decreases when a current is supplied to the lightemitting element, g (Q) is considered to be a monotonically decreasingfunction. The value of S can be considered to be the threshold of diodecharacteristics. Therefore, it can be explained that the threshold ofdiode characteristics of the light emitting element is shifted to ahigher voltage side when a current is supplied thereto.

From the formulas (1), (3), and (4), current density-voltagecharacteristics of the monitoring light emitting element and currentdensity-voltage characteristics of the displaying light emitting elementcan be represented by the following formulas, where Jo is a currentdensity (constant) of the monitoring light emitting element, Jp is acurrent density of a pixel, Qm is a total amount of charge in themonitoring light emitting element, Qp is a total amount of charge in thepixel, V is a voltage, and t is time.Jo=g(Qm)·V ^(f(t))  (5)Jp=g(Qp)·V ^(f(t))  (6)

From the formulas (5) and (6), the current density Jp in the pixel canbe represented by the following formula.Jp=Jo·g(Qp)/g(Qm)  (7)Since g (Q) is a monotonically decreasing function, the values of Jo andJp differ from each other when the monitoring light emitting element andthe displaying light emitting element have different currents. Forexample, more current flows through the monitoring light emittingelement than through the displaying light emitting element (i.e.,Qm>Qp), Jp is always larger than Jo.

The following consideration should be taken in order to ideally performthe CL drive for keeping the luminance of the displaying light emittingelement constant. First, the following formula can be obtained when theluminance of a pixel is L and current efficiency is η.L=η·Jp  (8)When the initial luminance is Lo and the initial current density is Jo,the current efficiency η is represented by the following degradationcurve where k is a rate constant and β is a parameter indicating theinitial degradation.η=(Lo/Jo)·exp {−(k·t)β}  (9)

As a result, the following formula (10) can be obtained from theformulas (8) and (9).L=Jp·(Lo/Jo)·exp {−(k·t)β}  (10)In order to maintain the luminance constant, L=Lo (constant) should besatisfied. Thus, when L=Lo is substituted in the formula (10), thefollowing formula (11) can be obtained.Jp=Jo·exp {(k·t)β}  (11)

That is, the CL drive can be achieved by increasing the value of Jp inaccordance with the formula (11). Finally, the following formula (12)can be obtained from the formulas (7) and (11).g(Qp)/g(Qm)=exp {(k·t)β}  (12)Thus, the CL drive can be achieved by controlling the values of Qp andQm so that g (Qp)/g(Qm) is close to exp {(k·t)β}.

When luminance decay is thus considered based on the amount of charge inthe light emitting element, the CL drive can be performed where theamount of charge in the displaying light emitting element is comparedwith the amount of charge in the monitoring light emitting element, andthe luminance of the displaying light emitting element is corrected soas to be constant.

Embodiment 1

In this embodiment, the test result of a display device of the inventionoperating at room temperature is described with reference to FIGS. 11Aand 11B. FIG. 11A shows the time-varying characteristics of current of alight emitting element (260 hours) while FIG. 11B shows the time-varyingcharacteristics of luminance of a light emitting element (260 hours). Inthe graphs of FIGS. 11A and 11B, a sample A is a panel having thecorrection function of the invention whereas a sample B and a sample Care panels having no correction function. The samples A and B are drivenwith constant voltage and the sample C is driven with constant current.

In the graphs of FIGS. 11A and 11B, the abscissa represents time (hour).The ordinate in FIG. 11A represents a normalized value of the actualcurrent (%) while the ordinate in FIG. 11B represents a normalized valueof the actual luminance (%).

In all the samples, the duty ratio of a monitoring light emittingelement is 100% whereas the duty ratio of a light emitting element isabout 64%. The monitoring light emitting element and the light emittingelement have the same total amount of current but differentinstantaneous currents.

FIG. 11A shows that the current of the sample A tends to increase withtime, the current of the sample B fluctuates considerably and tends todecrease with time, and the current of the sample C hardly fluctuatesand is kept substantially constant after the elapse of time.

The reason why the current of the sample A tends to increase with timeis because the monitoring light emitting element has a duty ratio of100% while the light emitting element has a duty ratio of 64% and thuschanges with time of the monitoring light emitting element progress morerapidly than change with time of the light emitting element.

FIG. 11B shows that the luminance of the sample A hardly fluctuates andis kept substantially constant after the elapse of time, the luminanceof the sample B fluctuates considerably and tends to decrease with time,and the luminance of the sample C hardly fluctuates though tends todecrease with time similarly to the sample B.

From the results shown in FIGS. 11A and 11B, it can be found that thatthe sample A using the invention has a constant luminance thoughincreasing current. This is because changes with time progress morerapidly by an increase +Δ in current. That is, the increase +Δ incurrent due to a correction function is almost equal to the decrease incurrent due to changes with time. Accordingly, the luminance of thesample A using the invention can be maintained substantially constant.

In view of the aforementioned operation, the display device of theinvention having a correction function, which has a constant luminance,can be called a constant luminance display device.

A driving method of the display device of the invention having acorrection function can be called a constant luminance drive method(constant brightness method, constant luminescence method, brightnesscontrol method, control brightness method, or bright control method).According to this driving method, as set forth above, an increase incurrent due to a correction function and a decrease in current due tochanges with time are obtained in advance, and a light emitting elementis driven at a voltage where the increase is equal to the decrease.

A rate of voltage rise for performing the constant luminance drivemethod is described below.

When the constant current drive is performed at an initial luminance L₀and a current density of J₀, a current efficiency η(t) that decreaseswith time is represented by the following formula as a function of timet.η(t)=L ₀ /J ₀ ×f(t)  (1)

It is known that f (t) can be represented by the following exponentialfunction.f(t)=exp {−(t/α)β}  (2)Note that α is a parameter indicating medium and long-term degradationand β is a parameter indicating initial degradation, which can beobtained experimentally.

Meanwhile, if the current density J changes with time t (i.e., J=J (t)),the luminance L can be represented by the following formula.L=η(t)×J(t)  (3)

Accordingly, in the case of performing the constant luminance drive, thefollowing formula (4) should be satisfied when L=L₀ (constant) issatisfied in the formula (3).L ₀=η(t)×J(t)  (4)

By substituting the formula (4) in the formula (1), the followingformula can be obtained.J(t)=J ₀ /f(t)  (5)

The formula (5) shows the phenomenon that in order to maintain theluminance constant, the current density should gradually increase fromJ₀ taking decrease in current efficiency into consideration. This isbecause the formula (2) shows that f(t) is a monotonically decreasingfunction.

In general, current density is proportional to power of voltage (x-thpower), and the following formula is thus obtained, where x is a powerdetermined by an element and C is a constant.J(t)=C×V ^(x)(t)  (6)

Accordingly, the following formula is obtained by substituting theformula (6) in the formula (5) and taking the formula (2) intoconsideration.V(t)=Const.×[exp(t/α)β]^(1/x)  (7)

The formula (7) shows how voltage should change to perform the constantluminance drive. Const. is a constant determined by an initial currentdensity J₀ and x (Const.=(J₀/C)^(1/x)). The constant luminance drive canbe achieved by taking this voltage rise into consideration.

Embodiment 2

The invention can be applied to a display device performing the constantcurrent drive as well.

In this embodiment, the level of changes with time is detected using aplurality of monitoring light emitting elements, and a video signal or apower supply potential is corrected based on the detection result,thereby changes with time of a light emitting element are corrected.Such a case is described with reference to FIG. 19.

This embodiment uses a plurality of (at least two) monitoring lightemitting elements, and two monitoring light emitting elements 1001 and1002 are provided herein. A constant current is supplied to one lightemitting element 1001 from a constant current source 1003 while aconstant current is supplied to the other light emitting element 1002from a constant current source 1004.

When the current supplied from the constant current source 1003 differsfrom the current supplied from the constant current source 1004, thetotal amounts of current flowing through the monitoring light emittingelements 1001 and 1002 are different from each other. Thus, changes withtime progress at different rates in the monitoring light emittingelements 1001 and 1002.

The monitoring light emitting elements 1001 and 1002 are connected to anarithmetic circuit 1005 that calculates a potential difference (voltagedifference) between one electrode of the monitoring light emittingelement 1001 and one electrode of the monitoring light emitting element1002.

A voltage calculated by the arithmetic circuit 1005 is supplied to avideo signal generating circuit 1006. In the video signal generatingcircuit 1006, a video signal supplied to each pixel is corrected basedon the voltage supplied from the arithmetic circuit 1005. According tothe aforementioned structure, changes with time of the light emittingelement can be corrected.

In the pixel shown in FIG. 6B, the gate electrode of the transistor 92is connected to the power supply line Vax with a constant potential,thereby the transistor 92 operates in a saturation region and lightemission or non-light emission of the light emitting element 13 iscontrolled by a video signal. In such a case, the video signal is notcorrected and the potential of the power supply line Vax may be changedbased on a voltage supplied from the arithmetic circuit 1005.

The power supply line Vax is connected to a power supply circuit 1007that corrects the potential of the power supply line Vax based on avoltage supplied from the arithmetic circuit 1005.

According to the display device of this embodiment having theaforementioned structure, correction can be performed in accordance withchanges with time.

It is preferable that a circuit for preventing fluctuations inpotential, such as a buffer amplifier, be provided between themonitoring light emitting element 1001 and the arithmetic circuit 1005and between the monitoring light emitting element 1002 and thearithmetic circuit 1005.

As a pixel having a configuration for performing the constant currentdrive, for example, a pixel using a current mirror circuit shown in FIG.20A, a pixel using another configuration shown in FIG. 20B, and the likemay be adopted.

A pixel using a current mirror circuit has transistors 1011 to 1014, acapacitor 1015, and a light emitting element 1016 (see FIG. 20A). Thecurrent mirror circuit is constituted by the transistors 1013 and 1014.A current flowing through the light emitting element 1016 is equivalentto a current flowing between a source and a drain of the transistor1014. A current flowing between a source and a drain of each of thetransistors 1013 and 1014 depends on charges held in the capacitor 1015.A pixel having another configuration has transistors 1021 to 1024, acapacitor 1025, and a light emitting element 1026 (see FIG. 20B).

Embodiment 3

A passive matrix display device using the invention is described withreference to FIG. 21. A passive matrix display device has a pixelportion 501 formed over a substrate, a column signal line driver circuit502 and a row signal line driver circuit 503 disposed at the peripheryof the pixel portion 501, and a controller 540 for controlling thecolumn signal line driver circuit 502 and the row signal line drivercircuit 503. The pixel portion 501 has x column signal lines C1 to Cxarranged in the column direction, y row signal lines L1 to Ly arrangedin the row direction, and a plurality of light emitting elementsarranged in matrix (x and y are natural numbers). The column signal linedriver circuit 502 and the row signal line driver circuit 503 areconstituted by an LSI chip and connected to the pixel portion 501 formedover the substrate through an FPC. A monitoring circuit 541 is providedover the same substrate as the pixel portion 501.

Operation of the passive matrix display device is briefly describedbelow. First, a row signal line L1 of the first row is selected. Morespecifically, the row signal line L1 is connected to a ground potentialthrough a switch 512. Then, when switches 508 to 511 of the columnsignal line driver circuit 502 are brought into a conductive state,currents from constant current sources 504 to 507 are supplied to lightemitting elements 524 to 527 arranged in the first row. Gray scaledisplay is achieved by the amount of current supplied from the constantcurrent sources 504 to 507 and the length of time during which currentsare supplied to the light emitting elements 524 to 527. When theswitches 508 to 511 are brought into a non-conductive state and the rowsignal line L1 is connected to Vcc through the switch 512, reverse biasvoltages are applied to the light emitting elements 524 to 527 of thefirst row. Such operation is repeated from the first row to the lastrow.

FIG. 16 shows a configuration example of the column signal line drivercircuit 502. A constant voltage source 601 has a function of generatinga constant voltage and uses a known constant voltage source with a smalltemperature coefficient such as a band gap regulator. A voltagegenerated from the constant voltage source 601 is converted into aconstant current with a small temperature coefficient by an operationalamplifier 602, a transistor 603, and a resistor 604. The convertedcurrent is reversed and copied by current mirror circuits constituted bytransistors 605 to 609 and resistors 614 to 618, and then supplied tothe column signal lines C1 to Cx through switches 610 to 613.

According to the display device of this embodiment, video data inputtedto the column signal line driver circuit 502 or a voltage generated fromthe constant voltage source 601 is corrected using the monitoringcircuit 541 in accordance with temperature changes and changes withtime, thereby the influences of changes in temperature and changes withtime can be prevented.

This application is based on Japanese Patent Application serial No.2004-152624 filed in Japan Patent Office on May 21, 2004, and JapanesePatent Application serial No. 2004-191833 filed in Japan Patent Officeon Jun. 29, 2004, the contents of which are hereby incorporated byreference.

1. A display device comprising: a first light emitting element; amonitoring second light emitting element; a constant current source forsupplying a constant current to the monitoring second light emittingelement; a circuit for outputting a potential corresponding to apotential which is inputted to said circuit; a capacitor connected to aninput terminal of the circuit; a first switch provided between a firstelectrode of the first light emitting element and an output terminal ofthe circuit; a second switch provided between the first electrode of thefirst light emitting element and an AC power supply; a third switchprovided between a first electrode of the monitoring second lightemitting element and the input terminal of the circuit; and a fourthswitch provided between the first electrode of the monitoring secondlight emitting element and the AC power supply.
 2. The display deviceaccording to claim 1, further comprising a control circuit for applyinga forward bias voltage to the first light emitting element and themonitoring second light emitting element while bringing the first switchand the third switch into a conductive state whereas the second switchand the fourth switch into a non-conductive state.
 3. The display deviceaccording to claim 1, further comprising a control circuit for applyinga reverse bias voltage to the first light emitting element and themonitoring second light emitting element while bringing the first switchand the third switch into a non-conductive state whereas the secondswitch and the fourth switch into a conductive state.
 4. The displaydevice according to claim 1, wherein the circuit is a buffer amplifier.5. The display device according to claim 1, wherein a second electrodeof the first light emitting element and a second electrode of themonitoring second light emitting element are maintained at a constantpotential.
 6. The display device according to claim 1, wherein the firstlight emitting element and the monitoring second light emitting elementare provided over the same substrate.
 7. An electronic device comprisingthe display device according to claim
 1. 8. A display device comprising:a first light emitting element; a monitoring second light emittingelement; a constant current source for supplying a constant current tothe monitoring second light emitting element; a circuit for outputting apotential corresponding to a potential which is inputted to saidcircuit; a capacitor connected to an input terminal of the circuit; afirst switch provided between a first electrode of the first lightemitting element and an output terminal of the circuit; a second switchprovided between the first electrode of the first light emitting elementand an AC power supply; a third switch provided between a firstelectrode of the monitoring second light emitting element and the inputterminal of the circuit; and a fourth switch provided between the firstelectrode of the monitoring second light emitting element and the ACpower supply, wherein the first light emitting element, the monitoringsecond light emitting element, the constant current source the circuit,the capacitor the first switch, the second switch, the third switch, andthe fourth switch are formed over a glass substrate.
 9. The displaydevice according to claim 8, further comprising a control circuit forapplying a forward bias voltage to the first light emitting element andthe monitoring second light emitting element while bringing the firstswitch and the third switch into a conductive state whereas the secondswitch and the fourth switch into a non-conductive state.
 10. Thedisplay device according to claim 8, further comprising a controlcircuit for applying a reverse bias voltage to the first light emittingelement and the monitoring second light emitting element while bringingthe first switch and the third switch into a non-conductive statewhereas the second switch and the fourth switch into a conductive state.11. The display device according to claim 8, wherein the circuit is abuffer amplifier.
 12. The display device according to claim 8, wherein asecond electrode of the first light emitting element and a secondelectrode of the monitoring second light emitting element are maintainedat a constant potential.
 13. The display device according to claim 8,wherein the first light emitting element and the monitoring second lightemitting element are provided over the same substrate.
 14. An electronicdevice comprising the display device according to claim
 8. 15. Anelectronic device comprising a display device, said display devicecomprising: a first light emitting element; a monitoring second lightemitting element; a constant current source for supplying a constantcurrent to the monitoring second light emitting element; a circuit foroutputting a potential corresponding to a potential which is inputted tosaid circuit; a capacitor connected to an input terminal of the circuit;a first switch provided between a first electrode of the first lightemitting element and an output terminal of the circuit; a second switchprovided between the first electrode of the first light emitting elementand an AC power supply; a third switch provided between a firstelectrode of the monitoring second light emitting element and the inputterminal of the circuit; and a fourth switch provided between the firstelectrode of the monitoring second light emitting element and the ACpower supply.
 16. The electronic device according to claim 15, furthercomprising a control circuit for applying a forward bias voltage to thefirst light emitting element and the monitoring second light emittingelement while bringing the first switch and the third switch into aconductive state whereas the second switch and the fourth switch into anon-conductive state.
 17. The electronic device according to claim 15,further comprising a control circuit for applying a reverse bias voltageto the first light emitting element and the monitoring second lightemitting element while bringing the first switch and the third switchinto a non-conductive state whereas the second switch and the fourthswitch into a conductive state.
 18. The electronic device according toclaim 15, wherein the circuit is a buffer amplifier
 19. The electronicdevice according to claim 15, wherein a second electrode of the firstlight emitting element and a second electrode of the monitoring secondlight emitting element are maintained at a constant potential.
 20. Theelectronic device according to claim 15, wherein the first lightemitting element and the monitoring second light emitting element areprovided over the same substrate.
 21. The electronic device according toclaim 15 is one selected from the group consisting of a television set,a digital camera, a digital video camera, a cellular phone, a portableinformation terminal, a portable game machine, a monitor for a computer,a computer, an audio reproducing device, and an image reproducingdevice.